I find the food/drink policies linked to in the previous post uniformly tortured and awful. If you’re going to let food in, let food in. Don’t make the policy more complex than the average international convention.
Maybe the best (most clear and concise) food policy I have encountered is from the University of Houston Libraries: "Food and drink may be consumed in most parts of the library – please dispose of your trash properly and in a timely manner. Please alert library staff if food or drink is spilled so that it can be cleaned up immediately…"